The objective of this research is to examine energy balance in low-birth weight infants by: 1) providing two alternative diets and, 2) measuring energy excretion and expenditure, and relating these to individual differences in the distribution of behavioral states. The subjects will be newborn, low-birth-weight infants, having no concurrent illness, fed either their mother's milk or a commercial formula. The infants will be reared in standard isolettes in a normal nursery environment. During the course of the investigation energy excretion will be estimated by bomb calorimetry of feces and urine; energy input will be estimated by weighing the infant's food and estimating the energy content of this by bomb calorimetry. Throughout the experiment measures will be made which will enable us to estimate behavioral state; these measures are: direct observation of behavioral states, respiration, heart rate. Measures of physical growth will be taken throughout the experiment. The data analysis will be directed at showing how growth is related to energy input, energy expenditure and energy excretion.